Laser radar apparatus

ABSTRACT

A laser radar apparatus includes a mixer  12  for converting the frequency of an electric signal outputted from a photodetector  11  into a baseband frequency by using a modulating signal generated by an oscillator  2.  The laser radar apparatus carries out a coherent integral of the electric signal whose frequency has been converted by the mixer  12,  and measures the quality of scatterers which exist in the atmosphere from a result of the integral. As a result, even when the S/N ratio of the light signal received by a receiving optical unit  8  is low, the laser radar apparatus can detect the quality of the scatterers.

FIELD OF THE INVENTION

The present invention relates to a laser radar apparatus that transmitsand receives laser light and measures the quality of scatterers in theatmosphere.

BACKGROUND OF THE INVENTION

By transmitting laser light into the atmosphere, a prior art laser radarapparatus receives laser light which has a Doppler frequency shift dueto the drift speed of scatterers in the atmosphere, and carries outheterodyne detection of the laser light and local light so as to detecta Doppler signal.

The prior art laser radar apparatus then acquires the drift speed of thescatterers from the frequency of the Doppler signal.

As mentioned above, although the prior art laser radar apparatus employsa method of detecting the Doppler shift frequency for the carrierfrequency of the laser light, it is known that this method exhibits weakcoherency of the Doppler signal. In other words, it is known that thecoherent time of the Doppler signal is short. For example, when thereceived light is light scattered from aerosols in the atmosphere, it isknown that the coherent time of the Doppler signal is of the order ofmicroseconds (μs).

Thus, when the coherent time of the Doppler signal is short, anincoherent integral of the Doppler signal is carried out so as to obtainan improvement in the S/N ratio of the received light, but it is knownthat it is difficult to effectively improve the S/N ratio of thereceived light even if an incoherent integral of the Doppler signal iscarried out.

In order to acquire a Doppler signal having a long coherent time, it isknown that an intensity modulation of the light signal is carried out byusing a frequency (for example, a modulation frequency which fallswithin a microwave frequency band) lower than that of the light signal,and what is necessary is just to detect the Doppler frequency for thismodulation frequency.

Following patent references 1 to 3 disclose laser radar apparatus forcarrying out an intensity modulation of a light signal by using amodulation frequency which falls within a microwave frequency band.

-   Patent reference 1 Japanese patent application publication No.    59-150299-   Patent reference 2 Japanese patent publication No. 51-29032-   Patent reference 3 Japanese patent application publication No.    2-25786

A problem with the prior art laser radar apparatus constructed asmentioned above is that while it can acquire a Doppler signal having along coherent time, when the S/N ratio of received light is low, thereis a possibility that the prior art laser radar apparatus is not able todetect the quality of scatterers in the atmosphere since it has no meansfor improving the S/N ratio of received light.

For example, when the scatterers are aerosols in the atmosphere and theprior art laser radar apparatus receives a light signal scattered fromaerosols and detects the drift speed (i.e., the air velocity) of theaerosols, the S/N ratio of the received light signal deterioratesgreatly as compared with the case where the scatterers are hard targets,such as automobiles.

The present invention is made in order to solve the above-mentionedproblems, and it is therefore an object of the present invention toprovide a laser radar apparatus that can detect the quality ofscatterers in the atmosphere even when the S/N ratio of received lightis low.

DISCLOSURE OF THE INVENTION

In accordance with the present invention, there is provided a laserradar apparatus including a frequency conversion means for convertingthe frequency of an electric signal outputted from a photoelectricconversion means into a baseband frequency by using a modulating signalgenerated by an oscillation means. The laser radar apparatus carries outa coherent integral of the electric signal whose frequency has beenconverted by the frequency conversion means, and measures the quality ofscatterers which exist in the atmosphere from a result of the integral.As a result, even when the S/N ratio of the light signal received by areceiving means is low, the laser radar apparatus can detect the qualityof the scatterers.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram showing a laser radar apparatus according toembodiment 1 of the present invention;

FIG. 2 is a block diagram showing the internal structure of a signalprocessing unit 13;

FIG. 3 is an explanatory drawing showing the waveform of a signal whichhas not been intensity-modulated by a light intensity modulator 3 andthe waveform of the signal which has been intensity-modulated by thelight intensity modulator 3;

FIG. 4 is an explanatory drawing schematically showing the visual fieldsof a transmitted beam and a received beam; FIG. 5 is a block diagramshowing a photoelectric conversion unit 30 of the laser radar apparatusaccording to embodiment 1 of the present invention;

FIG. 6 is a block diagram showing a laser radar apparatus according toembodiment 6 of the present invention;

FIG. 7 is a block diagram showing a laser radar apparatus according toembodiment 7 of the present invention; and

FIG. 8 is a block diagram showing the internal structure of a signalprocessing unit 13.

PREFERRED EMBODIMENTS OF THE INVENTION

Hereafter, in order to explain this invention in greater detail, thepreferred embodiments of the present invention will be described withreference to the accompanying drawings.

Embodiment 1

FIG. 1 is a block diagram showing a laser radar apparatus according toembodiment 1 of the present invention. In the figure, a light source 1transmits a light signal which consists of a continuous wave, and anoscillator 2 generates a modulating signal which consists of acontinuous wave. The oscillator 2 constitutes an oscillation means.

A light intensity modulator 3 performs an intensity modulation on thelight signal transmitted from the light source 1 using the modulatingsignal generated by the oscillator 2. The light intensity modulator 3constitutes a modulation means.

An optical transmitting amplifier 4 amplifies the light signalintensity-modulated by the light intensity modulator 3, and atransmitting optical unit 5 transmits the light signal amplified by theoptical transmitting amplifier 4 into the atmosphere by way of ascanning optical unit 6. The optical transmitting amplifier 4, thetransmitting optical unit 5, and the scanning optical unit 6 constitutea transmitting means.

When the transmitting optical unit 5 transmits a light signal into theatmosphere, a receiving optical unit 8 receives the light signal whichhas undergone a Doppler frequency shift due to scatterers contained inthe atmosphere by way of another scanning optical unit 7, an opticalreceiving amplifier 9 amplifies the light signal received by thereceiving optical unit 8, and an optical filter 10 removes unnecessaryfrequency components contained in the light signal amplified by theoptical receiving amplifier 9. The scanning optical unit 7, thereceiving optical unit 8, the optical receiving amplifier 9, and theoptical filter 10 constitute a receiving means.

By the way, the transmitting optical unit 5 and the receiving opticalunit 8 are so constructed as to have focal points at the same locationin the atmosphere by virtue of the operations of the scanning opticalunits 6 and 7.

A photodetector 11 detects an intensity-modulated component contained inthe light signal outputted from the optical filter 10, and outputs anelectric signal indicating the intensity-modulated component. Thephotodetector 11 constitutes a photoelectric conversion means.

A mixer 12 mixes the electric signal outputted from the photodetector 11and the modulating signal generated by the oscillator 2, and thenconverts the frequency of the electric signal into a baseband frequency.The mixer 12 constitutes a photoelectric conversion means.

A signal processing unit 13 carries out a coherent integral of theelectric signal whose frequency has been converted by the mixer 12 so asto detect the quality of the scatterers which exist in the atmospherefrom the integral result. The signal processing unit 13 constitutes adetection means.

A control unit 14 controls processes carried out by the scanning opticalunits 6 and 7 and the signal processing unit 13.

Optical fiber cables connect between the light source 1 and the lightintensity modulator 3, between the light intensity modulator 3 and theoptical transmitting amplifier 4, and between the optical transmittingamplifier 4 and the transmitting optical unit 5, respectively. Opticalfiber cables also connect between the receiving optical unit 8 and theoptical receiving amplifier 9, between the optical receiving amplifier 9and the optical filter 10, and between the optical filter 10 and thephotodetector 11, respectively. An electric wire cable connects betweenother components (for example, between the photodetector 11 and themixer 12). By using the optical fiber cables for connections between theabove-mentioned components, the degree of freedom of arrangement of thecomponents is improved as compared with a case where the light signal ismade to spatially propagate in the laser radar apparatus.

FIG. 2 is a block diagram showing the internal structure of the signalprocessing unit 13. In the figure, an A/D converter 21 converts theanalog electric signal whose frequency has been converted by the mixer12 into a digital signal. A coherent integrator 22 carries out acoherent integral of the digital signal outputted from A/D converter 21and simultaneously acquires a spectrum of the digital signal. An airvelocity detector 23 detects an air velocity in the direction of thetransmission of the light signal and in the vicinity of the focal pointsof the transmitting optical unit 5 and the receiving optical unit 8 fromthe spectrum acquired by the coherent integrator 22.

An electric wire cable connects among the A/D converter 21, the coherentintegrator 22, and the air velocity detector 23.

Next, the operation of the laser radar apparatus in accordance with thisembodiment of the present invention will be explained.

For convenience in explaining the operation of the laser radarapparatus, it is assumed that the scatterers are aerosols contained inthe atmosphere, and the laser radar apparatus acquires the Dopplerfrequency of the air velocity for the purpose of the detection of theair velocity (i.e., the detection of the drift speed of the aerosols).However, the laser radar apparatus can be applied to purposes other thanthe detection of the drift speed of the scatterers, for example, apurpose of acquiring the traveling speed of an automobile.

First, the light source 1 transmits a light signal which consists of acontinuous wave, and the oscillator 2 generates a modulating signalwhich consists of a continuous wave.

Assume that the modulating signal which the oscillator 2 generates has afrequency (i.e., a carrier frequency) equal to a frequency (for example,2 GHz) that falls within a microwave band usually used byelectromagnetic wave Doppler radars. The carrier frequency of thismodulating signal is far low as compared with the frequency (forexample, 200 THz) of the light signal transmitted from the light source1.

When receiving the light signal from the light source 1, the lightintensity modulator 3 performs an intensity modulation on the lightsignal using the modulating signal generated by the oscillator 2.

FIG. 3 shows the waveform of the light signal which has not beenintensity-modulated by the light intensity modulator 3 and the waveformof the light signal which has been intensity-modulated by the lightintensity modulator 3. FIG. 3A shows the waveform of the light signalwhich has not been intensity-modulated, and FIG. 3B shows the waveformof the light signal which has been intensity-modulated.

When receiving the light signal which has been intensity-modulated fromthe light-intensity modulator 3, the optical transmitting amplifier 4amplifies the light signal and the transmitting optical unit 5 transmitsthe light signal amplified by the optical transmitting amplifier 4 intothe atmosphere by way of the scanning optical unit 6.

After the light signal transmitted into the atmosphere is scattered bythe aerosols contained in the atmosphere, it is received by thereceiving optical unit 8 by way of the scanning optical unit 7.

The light signal received by the receiving optical unit 8 has undergonea Doppler frequency shift under the effect of the drift speed of thescatterers, i.e., the air velocity. In accordance with this embodiment1, since the light signal sent out by the light source 1 isintensity-modulated by using the modulating signal having a frequencythat falls within the microwave band, two types of the Doppler frequencyshift appear. One of them is a Doppler frequency shift for the carrierfrequency of the light signal, and the other one is a Doppler frequencyshift for the modulation frequency.

Here, the Doppler frequency shift f_(sc) for the carrier frequency ofthe light signal is expressed from the carrier frequency f_(c) of thelight signal, the propagation velocity c of light, and the drift speed vof the scatterers as follows:f _(sc)=(2v×f _(c))/c   (1)

On the other hand, the Doppler frequency shift f_(sm) for the modulationfrequency is expressed from the modulation frequency (i.e., the carrierfrequency of the modulating signal) f_(m), the propagation velocity c oflight, and the drift speed v of the scatterers as follows:f _(sm)=(2v×f _(m))/c   (2)

Since the focal points of the transmitting optical unit 5 and thereceiving optical unit 8 are placed at the same location, in the lightsignal received by the receiving optical unit 8, its componentsscattered from the focal points is in a dominant position.

When the receiving optical unit 8 receives the light signal, the opticalreceiving amplifier 9 amplifies the light signal and the optical filter10 removes unnecessary frequency components contained in the lightsignal amplified by the optical receiving amplifier 9.

When the optical filter 10 removes unnecessary frequency components fromthe light signal amplified by the optical receiving amplifier 9, thephotodetector 11 outputs an electric signal indicating theintensity-modulated component by directly detecting theintensity-modulated component contained in the light signal.

In other words, since the carrier frequency is removed from the lightsignal when the photodetector 11 directly detects theintensity-modulated component contained in the light signal, only theintensity-modulated component of the light signal is contained in thefrequency of the electric signal outputted from the photodetector 11.Therefore, the electric signal outputted from the photodetector 11 has afrequency which is shifted by only the Doppler frequency for themodulation frequency from the modulation frequency provided by the lightintensity modulator 3.

When receiving the electric signal from the photodetector 11, the mixer12 convert the frequency of the electric signal into a basebandfrequency by mixing the electric signal and the modulating signalgenerated by the oscillator 2.

In this case, the baseband frequency is equal to the difference betweenthe frequency of the modulating signal generated by the oscillator 2(i.e., the modulation frequency provided by the light intensitymodulator 3), and the frequency of the electric signal outputted fromthe photodetector 11 (i.e., the frequency which is shifted from themodulation frequency provided by the light intensity modulator 3 by onlythe Doppler frequency given by the equation (2)).

In other words, the electric signal outputted from the mixer 12 has theDoppler frequency corresponding to the equation (2) showing the airvelocity in the direction of the transmission of the light signal and inthe vicinity of the focal points of the transmitting optical unit 5 andthe receiving optical unit 8. In accordance with this embodiment 1, forconvenience in explaining the operation of the laser radar apparatus, aterm “Doppler signal” indicates a signal having a frequency equal to theDoppler frequency for the drift speed of the scatterers, and theelectric signal outputted from the mixer 12 is referred to as theDoppler signal from here on.

In accordance with this embodiment 1, the laser radar apparatus convertsthe frequency of the electric signal into a baseband frequency by usingthe single mixer 12, as previously mentioned. As an alternative, thelaser radar apparatus can carry out multiple-stage mixing, instead ofthe single-stage mixing, by using modulating signals each of which has acarrier frequency lower than the frequency of the modulating signalgenerated by the oscillator 2.

For example, the laser radar apparatus can mix the electric signaloutputted from the photodetector 11 and a modulating signal having afrequency of 1/3 f_(m) so as to convert the frequency of the electricsignal into an intermediate frequency, and further mixes an outputobtained by the previous mixing and another modulating signal having afrequency of 2/3 f_(m) so as to convert the frequency of the output intoa baseband frequency. In this case, although not shown in FIG. 1, thelaser radar apparatus is so simply constructed as to further include anoscillator for generating the modulating signal having the frequency of1/3 f_(m), an oscillator for generating the other modulating signalhaving the frequency of 2/3 f_(m), and another mixer other than themixer 12. By thus using the modulating signals having lower frequenciesfor the mixing, the laser radar apparatus offers an advantage of easilylengthening the coherency of the modulating signals, i.e., the coherenttimes of the modulating signals, and therefore making the Doppler signalcoherent.

In general, the coherent time of the Doppler signal is inverselyproportional to the frequency of the sent-out signal. Therefore, thecoherent time of the Doppler signal for the modulation frequencyprovided by the light intensity modulator 3 is far long as compared withthe coherent time of the Doppler signal for the carrier frequency of thelight signal.

For example, when an intensity modulation is performed on the lightsignal having a frequency of 200 THz (i.e., a wavelength of 1.5micrometers) with the modulation frequency of 2 GHz, the coherent timeof the Doppler signal for the modulation frequency is about 100,000times as long as that of the Doppler signal for the carrier frequency ofthe light signal. The carrier frequency fm of the modulating signal isdetermined from a required coherent time τ_(r) and a constant k asfollows:f _(m) =k/τ _(r)   (3)

Concretely, in the case where the light signal has a frequency of 200THz, the constant k can be set to k=2×10⁶ by taking into considerationthat the coherent time of the Doppler signal is about 1 microsecond.

When receiving the Doppler signal from the mixer 12, as mentioned above,the signal processing unit 13 carries out a coherent integral of theDoppler signal so as to detect the quality of the scatterers which existin the atmosphere from the integral result.

In other words, the A/D converter 21 of the signal processing unit 13carries out an A/D conversion of the Doppler signal outputted from themixer 12, and outputs an obtained digital signal to the coherentintegrator 22.

At this time, sampling intervals at which the digital signal is sampledare of an order corresponding to a desired distance resolution, and, forexample, is of the order of the focal length of the transmitting opticalunit 5 and the receiving optical unit 8.

When receiving the digital signal from the A/D converter 21, thecoherent integrator 22 carries out a coherent integral of the digitalsignal and simultaneously acquires a spectrum of the Doppler signal bycarrying out an FFT (Fast Fourier Transform) operation, etc. on thedigital signal.

According to this operation, the S/N ratio of the received light signalcan be improved in proportion to the number of samples. A reference(written by S. Goldman, Microwave System News & CommunicationTechnology, vol. 18-3, 44-52, (1988)) shows that an FFT operationperformed on a sampled signal yields a coherent integral of the signal.As an alternative, by using a method other than FFT, such as DFT(Discrete Fourier Transform), PP (Pulse Pair), or PPP (Poly Pulse Pair),the coherent integral of the digital signal can be carried out.

Instead of performing the coherent integral of the whole of the digitalsignal at once, the coherent integrator 2 can divide the digital signalinto some parts according to time gates each having a time widthdetermined by the reciprocal of the Doppler frequency which correspondsto a desired speed resolution, and further carries out a coherentintegral of coherent integral results each obtained for each time gateover all the time gates, thereby providing another advantage. Forexample, when the number of samples of the digital signal is A and eachsample of this digital signal is divided into B parts according to timegates, the number of times that operations are performed is equal toAlog₂A in case where FFT is performed on the whole of the digitalsignal. In contrast, in case where FFT is performed for every time gate,the number of times that operations are performed is equal to(A/B)×(Blog₂B)=Alog₂B. It is therefore apparent from A>B that theintegral result is obtained in a shorter time and with a fewer number oftimes that operations are performed in the case of the coherent integralof results obtained by performing an FFT operation for every time gate.In this case, since the time width of each time gate is set to thereciprocal of the Doppler frequency corresponding to the desired speedresolution, a frequency resolution for the FFT results is equal to theDoppler frequency corresponding to the desired speed resolution.Therefore, the desired speed resolution can be surely provided.

The air velocity detector 23 of the signal processing unit 13 detectsthe air velocity in the direction of the transmission of the lightsignal and in the vicinity of the focal point of the transmittingoptical unit 5 and the receiving optical unit 8 from the spectrumacquired by the coherent integrator 22.

As can be seen from the above description, according to this embodiment1, the laser radar apparatus includes the mixer 12 for converting thefrequency of an electric signal outputted from the photodetector 11 intoa baseband frequency by using the modulating signal generated by theoscillator 2 and is so constructed as to carry out a coherent integralof the electric signal whose frequency has been converted by the mixer12 so as to detect the quality of scatterers which exist in theatmosphere from the integral result. Even when the S/N ratio of a lightsignal received by the receiving optical unit 8 is low, the laser radarapparatus can detect the quality of scatterers which exist in theatmosphere.

In addition, according to this embodiment 1, the laser radar apparatusis so constructed as to divide an obtained digital signal into someparts according to time gates each having a time width determined by thereciprocal of a desired speed resolution, and to carry out a coherentintegral of the digital signal for each time gate and further carry outa coherent integral of coherent integral results each obtained for eachtime gate over all the time gates. Therefore, since the number of timesthat operations are performed is reduced, the laser radar apparatus canobtain the coherent integral result of the digital signal in a shorttime. The present embodiment offers another advantage of being able tosurely provide a desired speed resolution.

Furthermore, according to this embodiment 1, since when the oscillator 2generates a modulating signal, the laser radar apparatus determines thecarrier frequency f_(m) of the modulating signal so that the product ofthe carrier frequency f_(m) and coherent time τ_(r) of the modulatingsignal agrees with a constant k, the present embodiment offers a furtheradvantage of being able to carry out the coherent integral within ameasurement time period.

According to this embodiment 1, since the laser radar apparatus is soconstructed as to convert the frequency of the electric signal outputtedfrom the photodetector 11 into a baseband frequency by using the mixer12 for mixing the electric signal and the modulating signal generated bythe oscillator 2, the present embodiment offers another advantage ofbeing able to acquire an electric signal having the baseband frequencywithout complicating the structure of the laser radar apparatus.

In addition, according to this embodiment 1, the laser radar apparatusincludes two or more mixing stages for mixing modulating signals and theelectric signal, respectively. Therefore, the present embodiment offersa further advantage of being able to lengthen the coherent times of themodulating signals easily, thereby making the Doppler signal coherent.

According to this embodiment 1, while amplifying the light signalintensity-modulated and transmitting the light signal into theatmosphere, the laser radar apparatus amplifies a light signal receivedfrom the atmosphere and removes unnecessary frequency componentscontained in the received light signal. Therefore, the presentembodiment offers a further advantage of being able to improve theaccuracy of detection of the quality of the scatterers.

Furthermore, according to this embodiment 1, the laser radar apparatususes the photodetector 11 for directly detecting an intensity-modulatedcomponent contained in the light signal received by the receivingoptical unit 8. Therefore, the present embodiment offers anotheradvantage of being able to measure the quality of the scatterers with astable S/N ratio which is not influenced by fluctuations of thepolarized wave plane which a light signal propagating through an opticalfiber generally undergoes. Single mode fibers need not be used as theoptical fiber cables, and multimode fibers can be used as the opticalfiber cables.

Therefore, the present embodiment offers a further advantage of beingable to widen the visual fields of the transmitting optical unit 5 andthe receiving optical unit 8, and to easily make the focal points of thetransmitting optical unit 5 and the receiving optical unit 8 agree witheach other in the atmosphere.

In this embodiment 1, the carrier frequency of the signal transmitted bythe laser radar apparatus is strictly that of the light signal.Therefore, the laser radar apparatus can continuously hold superiorityin spatial resolution and local measurements over electromagnetic waveDoppler radar apparatus, for example.

In addition, in accordance with this embodiment 1, the laser radarapparatus is provided with the optical transmitting amplifier 4, theoptical receiving amplifier 9, and the optical filter 10. However, if asufficient S/N ratio of the received light signal is obtained even ifthe laser radar apparatus doesn't include these components, the laserradar apparatus need not include the components. However, when it isdifficult to measure the quality of the scatterers with a sufficient S/Nratio or when the receiving sensibility to the received light isinsufficient, it is desirable that the laser radar apparatus is providedwith at least one of the optical transmitting amplifier 4, the opticalreceiving amplifier 9, and the optical filter 10 in view of improvementsin the S/N ratio of the received light.

Furthermore, according to this embodiment 1, the laser radar apparatusdetects the Doppler frequency for signals having frequencies that fallwithin a microwave band, and can be applied to all methods aboutelectromagnetic wave radars using frequencies that fall within amicrowave band. Therefore, instead of assigning only one frequency tothe modulating signal generated by the oscillator 2, it is possible toassign two or more frequencies to the modulating signal and apply thelaser radar apparatus of this embodiment to measurement of the locationof a scatterer that is a hard target, such as an automobile, by using anFMCW method, which is known as one of electromagnetic wave radar systemsthat can sweep the two or more modulation frequencies.

In this case, the scanning optical units 6 and 7 are set so that thetransmitted beam from the transmitting optical unit 5 closely matcheswith the beam received by the receiving optical unit 8, in contrast tothe case as shown in FIG. 1. Thereby, if scatters exist in thetransmitted beam and in the received beam even if no scatterers exist atthe focal point of the scanning optical units shown in FIG. 1, lightsignals scattered by the scatterers can be received by the laser radarapparatus.

Embodiment 2

In accordance with above-mentioned embodiment 1, the laser radarapparatus converts the frequency of the electric signal into a basebandfrequency by using the mixer 12, as previously explained. In contrast, alaser radar apparatus in accordance with embodiment 2 is so constructedas to use an IQ detector circuit, instead of the mixer 12, and to makethe IQ detector circuit output an IQ video signal as a Doppler signal.

Thereby, the signal processing unit 13 can identify not only theabsolute value of the air velocity but the sign + or − of the airvelocity, i.e., whether the blowing winds are following or head ones.

Embodiment 3

In accordance with above-mentioned embodiment 1, the laser radarapparatus is equipped with the transmitting optical unit 5 and thereceiving optical unit 8, as previously explained. In contrast, a laserradar apparatus in accordance with embodiment 3 is so constructed as tohave one optical member provided with the functionalities of thetransmitting optical unit 5 and the receiving optical unit 8.

This single optical member can be provided with the functionality of anoptical circulator. Since the single optical member has a focal pointfor transmission and a focal location for reception that essentiallymatch with each other, the system can be easily constructed.

Embodiment 4

Although no mention is made in above-mentioned embodiment 1, each of theoptical fiber cables arranged between the light source 1 and thetransmitting optical unit 5 can be constructed of a single mode fiber,and each of the optical fiber cables arranged between the receivingoptical unit 8 and the photodetector 11 can be constructed of amultimode fiber.

FIG. 4A schematically shows the visual fields of a transmitted beam anda received beam at the time of using single mode fibers as the opticalfiber cables for both transmission and reception, and FIG. 4Bschematically shows the visual fields of a transmitted beam and areceived beam at the time of using single mode fibers as the opticalfiber cables for transmission and using multimode fibers as the opticalfiber cables for reception.

As shown in FIG. 4A, when single mode fibers are used as the opticalfiber cables for both transmission and reception, since the visualfields of the transmitted beam and the received beam can be narrowed,measurements with a high spatial resolution can be carried out while itis difficult to make the focal points of the transmitted beam and thereceived beam match with each other. In addition, when the speed ofscanning the transmitted beam and the received beam is increased by thescanning optical units 6 and 7, since the visual field of the receivedbeam moves to another position before the transmitted light is scatteredby scatterers and the received light is received, the receivingefficiency at the time of receiving the light scattered by thescatterers decreases.

On the other hand, as shown in FIG. 4B, when single mode fibers are usedas the optical fiber cables for transmission and multimode fibers areused as the optical fiber cables for reception, and the visual field forreception is made to be wider than that for transmission, it is possibleto easily make the focal points of the transmitted beam and the receivedbeam match with each other and to measure the quality of scatterers witha high spatial resolution. Since the visual field of the received beamis wide even if the speed of scanning the beams is increased, a higherreceiving efficiency can be provided as compared with the case wheresingle mode fibers are used as the optical fiber cables for bothtransmission and reception.

Embodiment 5

FIG. 5 is a block diagram showing a photoelectric conversion unit 30 ofa laser radar apparatus according to embodiment 1 of the presentinvention. In the figure, the photoelectric conversion unit 30 isdisposed instead of the photodetector 11 of FIG. 1. The photoelectricconversion unit 30 constitutes a photoelectric conversion means forconverting an intensity-modulated component contained in a light signaloutputted from an optical filter 10 into an electric signal.

A light source 31 outputs local light and a heterodyne detector 32carries out heterodyne detection of the light signal outputted from theoptical filter 10 and the local light outputted from the light source31. An envelope detector 33 detects an envelope of a detection signal ofthe heterodyne detector 32.

In accordance with this embodiment 5, the heterodyne detector 32 carriesout heterodyne detection of the light signal outputted from the opticalfilter 10 and the local light outputted from the light source 31, andthe envelope detector 33 detects the envelope of the detection signal ofthe heterodyne detector 32. As a result, the laser radar apparatusaccording to this embodiment outputs the detection result of theenvelope to a mixer 12 as an electric signal indicating anintensity-modulated component contained in the light signal, and canprovide a high SIN ratio under the effect of the heterodyne detection.However, since the laser radar apparatus according to this embodimentemploys the heterodyne detection, there is a necessity to use singlemode fibers as the optical fiber cables.

Although the light source 31 is disposed in addition to the light source1, as shown in FIG. 5, an optical divider, not shown in the figure, fordividing the light signal from the light source 1 into two parts can bedisposed, instead of the other light source 31, and one of them can beused as the local light.

Embodiment 6

FIG. 6 is a block diagram showing a laser radar apparatus according toembodiment 6 of the present invention. In the figure, since the samereference numerals as shown in FIG. 1 denote the same components asthose of embodiment 1 or like components, the explanation of thosecomponents will be omitted hereafter.

A pulse modulator 41 modulates a signal using a modulating signalgenerated by an oscillator 2 to generate a pulse having a time widthequivalent to a desired distance resolution. The pulse modulator 41 thenoutputs the pulse signal to a light intensity modulator 3. Electric wirecable connect between the oscillator 2 and the pulse modulator 41,between the pulse modulator 41 and the light intensity modulator 3, andbetween the pulse modulator 41 and a control unit 14, and the pulsemodulator 41 constitutes an oscillation means.

Next, the operation of the laser radar apparatus in accordance with thisembodiment of the present invention will be explained.

However, the explanation of the same portion as that of the laser radarapparatus according to above-mentioned embodiment 1 will be omittedhereafter.

When receiving a modulating signal which consists of a continuous wavefrom the oscillator 2, the pulse modulator 41 modulates a signal withthe modulating signal so as to generate a pulse having a time widthequivalent to a desired distance resolution, and then outputs the pulsesignal to the light intensity modulator 3.

Here, a relationship between the desired distance resolution and thetime width of the pulse is expressed from the desired distanceresolution d and the time width w of the pulse as follows:d=c×w/2   (4)

For example, when the desired distance resolution is 150 m, the timewidth of the pulse is set to 1 microsecond.

In FIG. 6, the scanning optical units 6 and 7 are so set that a beamtransmitted by a transmitting optical unit 5 closely matches with a beamreceived by a receiving optical unit 8. Furthermore, the transmittedbeam and the received beam are approximately placed in an unfocal stateof not having any focal point.

According to this embodiment 6, the light intensity modulator 3 carriesout an intensity modulation of the light signal transmitted from thelight source 1 using the pulse signal outputted from the pulse modulator41, and the transmitting optical unit 5 transmits the light pulse intothe atmosphere. Therefore, since the receiving optical unit 8 receiveslight signals scattered from two or more ranges in the atmosphere,Doppler frequencies of the Doppler signal for two or more time zonesrespectively corresponding to the two or more ranges can be detected. Asa result, the present embodiment offers an advantage of being able todetect the air velocity in the direction of the transmission of thelight pulse for each of the two or more ranges and hence to provide adistribution of the air velocity in the atmosphere.

Embodiment 7

FIG. 7 is a block diagram showing a laser radar apparatus according toembodiment 7 of the present invention. In the figure, since the samereference numerals as shown in FIG. 1 denote the same components asthose of embodiment 1 or like components, the explanation of thosecomponents will be omitted hereafter.

A sign sequence generator 42 generates a sign sequence which consists ofsigns each of which is + or −, and a phase modulator 43 modulates thephase of a modulation signal generated by the oscillator 2 using thesign sequence generated by the sign sequence generator 42. The signsequence generator 42 and the phase modulator 43 constitute anoscillation means.

Electric wire cables connect between the sign sequence generator 42 andthe phase modulator 43, between the sign sequence generator 42 and asignal processing unit 13, between the phase modulator 43 and anoscillator 2, and between the phase modulator 43 and a light intensitymodulator 3.

FIG. 8 is a block diagram showing the internal structure of the signalprocessing unit 13. In the figure, since the same reference numerals asshown in FIG. 2 denote the same components as those of the signalprocessing unit 13 of embodiment 1 or like components, the explanationof those components will be omitted hereafter.

A demodulator 24 performs a demodulation process on a digital signaloutputted by an A/D converter 21 based on the sign sequence generated bythe sign sequence generator 42.

Next, the operation of the laser radar apparatus in accordance with thisembodiment of the present invention will be explained.

However, the explanation of the same portion as that of the laser radarapparatus according to above-mentioned embodiment 1 will be omittedhereafter.

The sign sequence generator 42 generates a sign sequence which consistsof signs each of which is + or −. The sign sequence simply has a sharppeak at a delay time of 0 in an autocorrelation function thereof andincludes a set of signs periodically repeated. An M sequence signal canbe provided as an example of the sign sequence. Hereafter, the operationof the laser radar apparatus will be explained by assuming that thelaser radar apparatus uses an M sequence signal as the sign sequence.

When the sign sequence generator 42 generates an M sequence signal, thephase modulator 43 modulates the phase of the modulation signalgenerated by the oscillator 2 using the M sequence signal.

In other words, the phase modulator 43 performs a phase modulation witha phase of 0 or π on the modulation signal based on the M sequencesignal, and transmits this signal as a modulated signal to the lightintensity modulator 3. Each bit of the phase-modulated signal has a timewidth equivalent to a desired distance resolution.

Thereby, a beam transmitted by a transmitting optical unit 5 becomes asignal having a duty ratio of 100%, which is pseudo-random-modulatedwith the M sequence signal.

In FIG. 7, scanning optical units 6 and 7 are so set that the beamtransmitted by the transmitting optical unit 5 closely matches with abeam received by a receiving optical unit 8. Furthermore, thetransmitted beam and the received beam are approximately placed in anunfocal state of not having any focal point.

Therefore, rays of light scattered by two or more ranges in theatmosphere are superimposed on and contained in the beam received by thereceiving optical unit 8.

When receiving a Doppler signal from the mixer 12, the signal processingunit 13 carries out a coherent integral of the Doppler signal andmeasures the quality of scatterers which exist in the atmosphere fromthe integral result, like that of the laser radar apparatus ofabove-mentioned embodiment 1.

In other words, the A/D converter 21 of the signal processing unit 13carries out A/D conversion of the Doppler signal outputted from themixer 12, and outputs a digital signal to the demodulator 24.

The demodulator 24 of the signal processing unit 13 performs ademodulation process on the digital signal outputted from the A/Dconverter 21 based on the sign sequence generated by the sign sequencegenerator 42.

To be more specific, the demodulator 24 selects one range in theatmosphere which is a target for measurement of the air velocity, andsets a delay time, i.e., a time that lapses until transmitted lightscattered by scatterers located in this selected range is received asreceived light since the transmitted light has reached the range to τ.Hereafter, the selected range is referred to as the measurement range.

The demodulator 24 multiplies each sample of the digital signal by +1 or−1 according to the M sequence signal generated by the sign sequencegenerator 42 at the timing defined by the M sequence signal which hasbeen delayed by only the delay time τ. As a result, the phase of theDoppler signal from the measurement range is demodulated and a coherentsignal having the Doppler frequency in the measurement range can beacquired.

In this case, a range side lobe level from the other ranges in theatmosphere becomes 1/N′, assuming that the signal from the measurementrange has a strength of 1 after it has been demodulated and the numberof bits of the M sequence signal is N′. Therefore, by increasing thenumber N′ of bits of the M sequence signal, the range side lobe levelcan be reduced to a negligible value.

The demodulated signal obtained by the demodulator 24 is sent to thecoherent integrator 22. At this time, sampling intervals at which thedemodulated signal is sampled are of an order corresponding to a desireddistance resolution.

When receiving the demodulated signal from the demodulator, the coherentintegrator 22 carries out a coherent integral of the demodulated signaland simultaneously acquires a spectrum of the Doppler signal by carryingout an FFT operation, etc. on the demodulated signal.

The air velocity detector 23 detects the air velocity in the directionof transmission of the light signal and in the measurement range fromthe spectrum acquired by the coherent integrator 22.

As can be seen from the above description, according to this embodiment7, the laser radar apparatus transmits and receives light which consistsof a modulated pseudo CW having a duty ratio of 100% and performs ademodulation process on the received light. Therefore, the presentembodiment offers an advantage of being able to provide a desired S/Nratio in a short time and to carry out detection of the air velocitywith a desired distance resolution.

In accordance with this embodiment 7, the laser radar apparatus selectsone measurement range and detects the air velocity for the selectedmeasurement range, as previously mentioned. As an alternative, the laserradar apparatus can select two or more measurement ranges, and two ormore demodulators 24, two or more coherent integrators 22, and two ormore air velocity detectors 23 can be disposed in parallel with oneanother for the two or more measurement ranges, respectively. Thisvariant offers an advantage of being able to acquire a distribution ofthe air velocity in the atmosphere.

As previously mentioned, the laser radar apparatus in accordance withany one of above-mentioned embodiments 1 to 7 is applied to the casewhere the quality of scatterers to be measured is the drift speed ofscatterers. However, the laser radar apparatus in accordance with thepresent invention is not limited to this example. For example, the laserradar apparatus in accordance with the present invention can be alsoapplied to a case where the quality of scatterers is the location orreflectance of scatterers.

For example, when the scatterers in the atmosphere are a hard target,such as an automobile, the laser radar apparatus only has to change thedelay time τ of the demodulator 24, determine the delay time τ at whichan integral value in the coherent integral result is maximized, anddetects the location of the scatterer from a distance corresponding tothe integral value. Furthermore, the laser radar apparatus only has todetect the reflectance of the scatterer from the integral value in thecoherent integral result.

Industrial Applicability

As mentioned above, the laser radar apparatus according to the presentinvention is suitable for transmitting and receiving laser light,detecting the quality of scatterers, and determining the drift speed,location and reflectance of the scatterers.

1. A laser radar apparatus comprising: an oscillation means forgenerating a modulating signal; a modulation means forintensity-modulating a light signal by using the modulating signalgenerated by said oscillation means; a transmitting means fortransmitting the light signal intensity-modulated by said modulationmeans into an atmosphere; a receiving means for receiving a light signalfrom the atmosphere when said transmitting means transmits the lightsignal into the atmosphere; a photoelectric conversion means forconverting an intensity-modulated component contained in the lightsignal received by said receiving means into an electric signal; afrequency conversion means for converting a frequency of the electricsignal outputted from said photoelectric conversion means into abaseband frequency by using the modulating signal generated by saidoscillation means; and a detection means for carrying out a coherentintegral of the electric signal whose frequency has been converted bysaid frequency conversion means so as to detect a quality of scattererswhich exist in the atmosphere from a result of the coherent integral. 2.The laser radar apparatus according to claim 1, characterized in thatsaid oscillation means generates the modulating signal which consists ofa continuous wave.
 3. The laser radar apparatus according to claim 1,characterized in that said oscillation means generates a pulse signalhaving a time width equivalent to a desired distance resolution as themodulating signal.
 4. The laser radar apparatus according to claim 1,characterized in that said oscillation means generates the modulatingsignal by using a sign sequence which consists of signs each of whichis + or −.
 5. The laser radar apparatus according to claim 1,characterized in that said detection means converts the analog electricsignal whose frequency has been converted by said frequency conversionmeans into a digital signal, and divides the digital signal into partswith time gates each of which has a time width determined by areciprocal of a desired speed resolution, and carries out a coherentintegral of the digital signal for each of the time gates, whilecarrying out a coherent integral of results of the coherent integral forthe time gates.
 6. The laser radar apparatus according to claim 1,characterized in that when generating the modulating signal, saidoscillation means determines a carrier frequency of the modulatingsignal so that a multiplication of the carrier frequency and a coherenttime of the modulating signal agrees with a predetermined value.
 7. Thelaser radar apparatus according to claim 1, characterized in that saidfrequency conversion means is provided with a mixer for mixing theelectric signal outputted from said photoelectric conversion means andthe modulating signal generated by said oscillation means.
 8. The laserradar apparatus according to claim 7, characterized in that when two ormore oscillation means are disposed, said frequency conversion meansmixes modulating signals generated by said two or more oscillation meansand the electric signal, respectively.
 9. The laser radar apparatusaccording to claim 1, characterized in that one optical member isprovided with a function of said transmitting means and a function ofsaid receiving means.
 10. The laser radar apparatus according to claim1, characterized in that said transmitting means amplifies the lightsignal intensity-modulated by said modulation means and then transmitsthe amplified light signal into the atmosphere, and said receiving meansamplifies the light signal received thereby from the atmosphere and thenremoves unnecessary frequency components contained in the received lightsignal.
 11. The laser radar apparatus according to claim 1,characterized in that said photoelectric conversion means is providedwith a photodetector for detecting an intensity-modulated componentcontained in the light signal received by said receiving means.
 12. Thelaser radar apparatus according to claim 1, characterized in that anoptical fiber cable is provided for connecting between said modulationmeans and said transmitting means and an optical fiber cable is providedfor connecting between said receiving means and said photoelectricconversion means.
 13. The laser radar apparatus according to claim 12,characterized in that all or part of said optical fiber cables aremultimode fibers.
 14. The laser radar apparatus according to claim 12,characterized in that said optical fiber cable that connects betweensaid modulation means and said transmitting means is a single modefiber, and said optical fiber cable that connects between said receivingmeans and said photoelectric conversion means is a multimode fiber. 15.The laser radar apparatus according to claim 1, characterized in thatsaid photoelectric conversion means is provided with a heterodynedetector for carrying out a heterodyne detection of both the lightsignal received by said receiving means and local light, and an envelopedetector for detecting an envelope of a detection signal from saidheterodyne detector.